the air filter on the Koleos is due for replacement (power cut in "high" revs (3000+)). Since we will not sell this car and I like custom things, I'm wondering if it could be an issue if I ditch the factory airbox and inlet tract to put some aftermarket washable filter in place. See the picture :

I recognize it's a fantasy but could it have negative effects that I'm not aware of ? boost threshold is as low as 1700rpm.

Here they have a reputation for going through air sensor and turbo quickly, but my guess is poor maintenance (I think OEM recommendation have very long OCI). Some cracked head reports too, from time to time, but mostly on early engines.

(this is to be taken with a grain of salt, on forums or car magazines most of the time you only see the worst, and not the perfectly happy users with perfectly running engine haha)

Réunion Island, a French department in the Indian Ocean, is known for its volcanic, rainforested interior, coral reefs and beaches. Its most iconic landmark is Piton de la Fournaise, a climbable active volcano standing 2,632m (8,635 ft.). Piton des Neiges, a massive extinct volcano, and Réunion’s 3 calderas (natural amphitheaters formed by collapsed volcanoes), are also climbing destinations.

"The evidence demands a verdict".(Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."

Réunion Island, a French department in the Indian Ocean, is known for its volcanic, rainforested interior, coral reefs and beaches. Its most iconic landmark is Piton de la Fournaise, a climbable active volcano standing 2,632m (8,635 ft.). Piton des Neiges, a massive extinct volcano, and Réunion’s 3 calderas (natural amphitheaters formed by collapsed volcanoes), are also climbing destinations.

The majority of aftermarket filters are less efficient than factory paper filters so cause more damage to turbo, bores, rings etc. If your filter is clogging too quickly then the best solution would be to use an air filter & housing with more surface area to allow greater life.